What We’re DoingThe final circuit is a bit of a test. We combine what we learned about using transistors in CIRC03 to control a relay. A relay is an electrically controlled mechanical switch. Inside the little plastic box is an electromagnet that, when energized, causes a switch to trip (often with a very satisfying clicking sound). You can buy relays that vary in size from a quarter of the size of the one in this kit up to as big as a fridge, each capable of switching a certain amount of current. They are immensely fun because there is an element of the physical to them. While all the silicon we’ve played with to this point is fun sometimes you just want to wire up a hundred switches to control something magnificent. Relays give you the ability to dream it up then control it with your Arduino. Now to using today’s technology to control the past. (The 1N4001 diode is acting as a flyback diode, for details on why it’s there visit: http://ardx.org/4001)

Code(no need to type everything in just)File > Examples > Digital > Blink (example from the great arduino.cc site check it out for other ideas)/*
Blink
Turns on an LED on for one second, then off for one second, repeatedly.
The circuit:
* LED connected from digital pin 13 to ground.
* Note: On most Arduino boards, there is already an LED on the board
connected to pin 13, so you don’t need any extra components for this example.

Created 1 June 2005
By David Cuartielles
http://arduino.cc/en/Tutorial/Blink
based on an orginal by H. Barragan for the Wiring i/o board
*/

// the loop() method runs over and over again,
// as long as the Arduino has power

void loop()
{
digitalWrite(ledPin, HIGH); // set the LED on
delay(1000); // wait for a second
digitalWrite(ledPin, LOW); // set the LED off
delay(1000); // wait for a second
}

Not Working?(3 things to try)

Nothing HappensThe example code uses pin 13 and we have the relay connected to pin 2. Make sure you made this change in the code.

No Clicking SoundThe transistor or coil portion of the circuit isn’t quite working. Check the transistor is plugged in the right way.

Not Quite WorkingThe included relays are designed to be soldered rather than used in a breadboard. As such you may need to press it in to ensure it works. (and it may pop out occasionally)

Making it Better?

Watch the Back-EMF Pulse
Replace the diode with an LED. You’ll see it blink each time it “snubs” the coil voltage spike when it turns off.

Controlling a Motor
In CIRC-03 we controlled a motor using a transistor. However if you want to control a larger motor a relay is a good option. To do this simply remove the red LED, and connect the motor in its place (remember to bypass the 560 Ohm resistor)

Controlling Motor Direction
A bit of a complicated improvement to finish. To control the direction of spin of a DC motor we must be able to reverse the direction of current flow through it. To do this manually we reverse the leads. To do it electrically we require something called an h-bridge. This can be done using a DPDT relay to control the motor’s direction, wire up the following circuit. It looks complicated but can be accomplished using only a few extra wires. Give it a try.